This is a R21 developmental grant application in response to NCI PA- 01-010, "Exploratory studies in cancer detection, prognosis and prediction." The overall goal is to produce a CPT-11 (Irinotecan) carboxylesterase assay that could be used to predict treatment outcome and/or toxicity for patients on CPT-11 therapy for colorectal cancer. CPT-11 is a semi-synthetic pro-drug that is activated by hydrolysis in vivo to SN-38. SN-38 is a potent inhibitor of topoisomerase I and therapy inhibits cell growth. Another important metabolite called APC is also hydrolyzed to SN-38. The specific carboxylesterases responsible for the hydrolytic activation of CPT-11 and APC to SN-38 are not known. Two human carboxylesterases, hCE-1 and hCE-2 is highly expressed in intestine, which may be related to the major toxic complication of CPT- 11 therapy, diarrhea. Our hypothesis is that the tissue and cell-specific expression of CPT-1 and APC carboxylesterases may be an important determinant of the therapeutic outcome and toxicity associated with the APC carboxylesterases. Analysis of carboxylesterase-like genes in GenBank and preliminary observations in tumor cell lines suggest that there may be other carboxylesterases that could catalyze the hydrolysis of CPT-11 and APC. Proteomics and PCR methodologies will be used to screen for carboxylesterases and kinetic analysis with CPT-11 and APC as substrates will be performed with isolated or expressed enzymes. The second scientific aim of the grant is to develop and validate assays employing activity assays, gel electrophoresis or PCR methodologies for analysis of CPT-11 carboxylesterase expression in tumor and normal colon tissue from patients treated with CPT-11. A pilot study will be performed with tumor and normal tissue collected from patients at the Indiana University Cancer Center who presented with metastatic disease at diagnosis. After surgery the patients will be treated with 5- fluorouracil, Leucovorin and CPT-11. The response to CPT-11 therapy and associated toxicity will be compared to carboxylesterase expression in tumor and normal colon tissue. If there is a positive correlation, a future multi-institutional study will be proposed.